This invention relates generally to water vapor delivery systems and, more particularly, to a method and apparatus for generating dry vapor with minimal liquid stagnation, thereby limiting growth of bacteria and algae.
Semiconductor circuits commonly use aluminum films as a conductor material. The aluminum films are etched with chlorinated gases to produce connections within the circuits. However, chlorine often is retained within the sidewalls of the aluminum films and reacts with absorbed water vapor to form hydrochloric acid as soon as the semiconductor wafer is brought into ambient atmosphere from the controlled chamber environment after processing. The hydrochloric acid reacts with the aluminum film to form aluminum chlorides, which in turn react with the absorbed water vapor to produce more hydrochloric acid. This self-sustaining reaction corrodes the aluminum films and renders the affected semiconductor chip worthless.
One method of removing chlorine prior to transferring the wafer from the controlled chamber environment is to strip the photoresist from the wafer by ashing. Typically, a nitrogen and oxygen plasma is used with a wafer elevated to temperatures between 180.degree. and 260.degree. C. This technique has met with some success. Additional benefits are obtained by mixing in ammonia or carbon tetrafluoride, or both, as well. Corrosion protection of up to 24 hours has been obtained, but these results can not always be guaranteed.
Other methods of chlorine removal have used separate wafer washing modules, or wafer washing modules integrated with the etching system, to wash and rinse the etched wafers with water-based solutions as soon as possible after the etching process to remove the chlorine. However, these washing modules are generally large and are relatively costly.
Finally, a photoresist stripper process that includes water vapor plasma, sometimes combined with nitrogen and oxygen, has been used. The water vapor in the stripper is disassociated by the plasma so that the hydrogen will combine with the chlorine to form gaseous hydrochloric acid, which can be satisfactorily pumped from the stripper chamber. This technique has shown to be virtually totally reliable and can deliver corrosion protection in excess of 72 hours. However, the water vapor used in this method generally does not have a sufficiently high vapor pressure at ambient temperature to provide a supply of gas at a rate that will meet the requirements of the stripper. Also, because the vapor pressure of water varies with temperature, there is a need for a water vapor generator that can consistently supply water vapor at a rate and pressure that will meet or exceed the requirements of the stripper.
Previously, water vapor delivery systems used with the photoresist strippers included a relatively large assembly of readily available water and nitrogen valves, regulators, gauges, interconnecting tubing and fittings, all packaged in an externally mounted cabinet. Although the parts used to make these assemblies are easily obtained, the assemblies are bulky and expensive. Their bulkiness prevents the assemblies from being incorporated into a VDS (vapor delivery system) enclosure, and the cost of the assembly significantly increases the total cost of the water vapor delivery system. Furthermore, such assemblies include a significant "dead leg," the distance between the constantly circulating water source and the water vapor generator, that increases the risk of bacteria and algae growth in the assembly. The length of this dead leg can be six and one-half feet or more.
Additionally, early water vapor delivery systems incorporated a commercially available boiler, which is typically a large glass bottle, containing a liter or more of deionized water, captured within a temperature-controlled enclosure. A substantial portion of this container is purposely left empty to accumulate water vapor, which is drawn off through a mass flow controller. The water vapor flowing through the mass flow controller must be dry, i.e., be unsaturated, and contain relatively few water drops, preferably no drops at all. Accordingly, a substantial distance must be maintained between the water surface and the vapor outlet of the container to prevent water drops due to rising water bubbles from entering the vapor outlet. However, the bulky size of conventional boilers requires them to be located outside of present etching system enclosures. This necessitates running relatively long vapor lines to the strip chamber which exposes the heated and insulated vapor lines unduly to potential abuse by personnel working about the equipment.